Seismometers frequently experience spontaneous internal transient disturbances. This tends to occur most frequently with new instruments and to lesser degree when an instrument is first installed, moved or otherwise disturbed. These disturbances can appear as spikes in a time domain signal data. Spikes having amplitudes sufficiently larger than the concurrent seismic signal can be distinguished from natural events by their shape. However, even extremely low amplitude disturbances too small to be distinguished from low level seismic background activity in the time domain, can become apparent in the frequency domain, obscuring the power spectral density of the real seismic signal in the frequency range where it is quietest.
The disturbances occur when mechanical stresses in the components of the seismometer are spontaneously relieved; causing a mechanical shock that is interpreted as seismic signal. The frequency of occurrence of disturbances (often called pings and pops) may decline over time (months to years) as stresses inherent in the assembly are permanently relieved. Stresses induced by environmental changes or moving parts can build and be spontaneously relieved repetitively through the operational life of the seismometer. Conventional approaches to minimize pings or pops include minimizing static mechanical stresses, aging of components, temperature cycling of components and assemblies to relieve internal stresses, and careful assembly to both minimize stresses and the likelihood that stresses would be relieved.
A broadband seismometer typically uses an adjustable mass positioning mechanism operated by an electrical motor or a manually adjusted screw to position an inertial mass to a measurement null point to compensate for the inertial mass moving from the null point due to environmental changes or mis-calibration. This mechanism has moving parts that can be a source of pings and pops.
There is a need to provide mass positioning mechanisms that are less susceptible to spontaneous micro-mechanical movement that can be realized relatively economically inside the seismic sensor without significantly increasing the volume of the sensor enclosure.